Such structures as ships, bridges, vehicles, aircraft and machine tools are constructed mainly of metals such as iron, aluminum and magnesium, and alloys thereof. If a repeated load is imposed on any of these metallic structures, a crack may be developed particularly in a stress-concentrated portion of the metallic structure due to metal fatigue. Since such a fatigue crack of the structure develops gradually with the lapse of time, the structure is inspected for cracks periodically or non-periodically.
Such a crack inspection is generally performed by visual inspection. In a special case, a precise inspection is performed using a device such as, for example, an ultrasonic flaw detector. For example, in JP 4-169836A is disclosed a method wherein a strain varying range of a structure to be inspected is determined from the time when a fine line breaks, thereby accurately predicting the time when a fatigue damage occurred. In JU 1-180757A is disclosed a method wherein a ribbon-like conductive film is formed in a crack detecting portion of a structure, a conduction detector is connected to both ends of the ribbon-like conductive film and fracture of the conductive film simultaneous with cracking of the film is detected by the conduction detector, thereby detecting the occurrence of a fatigue crack in an early stage.
However, all of the above precise inspection methods require the installation of measuring devices and thus cost high. Besides, skill is needed because the handling of measuring devices is troublesome. Further, in case of inspecting a narrow place or a place where various members are installed in a complicated state, it is difficult or impossible to use the measuring devices.
In an effort to solve such problems involved in the inspection methods using measuring devices, an inspection method is disclosed in JP 10-267866A (prior art 1) wherein a coating layer having dispersed therein glass capsules with a visualizing liquid sealed therein is formed on the surface of a structure to be inspected and consequently a crack is developed in the coating layer along a crack developed in the structure, whereby there occurs breakage of the glass capsules in the coating layer and the visualizing liquid flows out onto the surface of the coating layer, thus permitting detection of the cracked portion.
In U.S. Pat. No. 5,534,289 (prior art 2) is disclosed an inspection method wherein a first coating layer having dispersed therein microcapsules with a visualizing liquid sealed therein is formed on the surface of a structure and a second coating layer different in color from the visualizing liquid is formed on the first coating layer, allowing a crack if occurred in the structure to be propagated to the first and second coating layers, resulting in rupture of the microcapsules, then the visualizing liquid flows out from the microcapsules, further flows along the cracks and is sensed upon arrival at the surface of the second coating layer, thereby detecting the occurrence of the crack in the structure. A red dye is used therein as the visualizing liquid.
In the methods disclosed in the prior arts 1 and 2, each using a visualizing liquid, the visualizing liquid oozes out to the surface of a coating layer along a crack developed in a structure and the presence of the crack is checked by seeing the visualizing liquid. However, there has been the problem that the visualizing liquid which has oozed out to the coating layer surface fades little by little with the lapse of time and gradually becomes invisible.
Moreover, in the prior art 2, a red dye of an azo or anthraquinone compound is microencapsulated and is mainly used as a visualizing liquid, and a coating layer with the microcapsules dispersed therein is applied to the surface of a structure. However, in case of using this visualizing liquid, the red visualizing liquid which had appeared with the occurrence of a crack in the structure fades little by little under the influence of ultraviolet light for example and gradually becomes invisible.